1. Field of the Invention
The present invention relates to a magnetic brush developing apparatus for developing an electrostatic latent image, which is used in an electrophotographic recording apparatus or the like.
Widely known as a developing method applicable to a developing apparatus is a two-component magnetic brush developing method using a toner or a colored powder, and a carrier or a magnetic powder as a developer.
The carrier used therein imparts an electrostatic charge to the toner through a triboelectrification or the like thereof, and the toner is held by the electrostatic force for conveyance.
Also, the developing roller functions to convey the developer to the developing section and holds the carrier by the magnetic force for conveyance, and thus the magnetic properties of the developing roller have a great influence on the image quality.
2. Description of the Related Art
The principle of the two-component magnetic brush developing method relevant to the present invention, will now be described with reference to FIG. 18.
A developing roller 1 is constituted of a magnet 1b having a plurality of magnetic poles, and a rotational sleeve 1a which rotatably covers the surface of the magnet 1b.
The magnet 1b is usually fixed, and a developer 4 is caught on the surface of the rotational sleeve 1a, through the magnetic force exerted by the magnet 1b, and carried to a developing section 9 in which a photosensitive drum 3 confronts the developing roller 1a, through the rotation of the rotational sleeve 1a.
The developer restriction member 2, i.e., a blade is arranged opposite to the developing roller 1 (rotational sleeve 1a) to restrict the amount of the developer 4 to be carried to the developing section 9.
The amount of the developer 4 passing through the developer restriction member 2 is determined by the interval (a blade gap) between the developing roller 1 and the developer restriction member 2, and the amount by which the developer head has been raised. Accordingly, the amount of the developer 4 which has passed through the developer restriction member 2 is determined by the height of the developer head.
The image quality is also influenced by the height of the developer head. Namely if the height of the developer is comparatively low and the amount of the developer to be carried to the developing section 9 is thus reduced, sufficient toner for a development is not supplied, which results in a deterioration of the image density. Conversely, if an excessive amount of toner is supplied, the image is unstable.
Namely, a variation in the height of the developer head may cause an instability of the image and a deterioration of the image quality. Accordingly, to eliminate any unevenness of the image quality, it is necessary to maintain the quantity of developer 4 passing through the blade gap of a constant value.
Moreover, even though the blade gap is constant, the quantity of the developer 4 passing through the blade gap is determined by the amount by which the developer resting on the developing roller 1 is raised in the developer restriction member facing section 6.
Therefore, to obtain a firm image quality without unevenness, that condition of the raising of developer head on the developing roller 1 in the developer restriction member facing section 6 should be always the same.
The condition of the raising of the developer is determined by the vertical magnetic flux density at a position opposite to the developer restriction member 2 on the developing roller 1, and as a result, the vertical magnetic flux density at the opposing position must be constant.
FIG. 19 shows the distribution of the vertical component of the magnetic flux density of the conventional developing roller 1. Note, in this specification, the "vertical component of the magnetic flux density" will be hereinafter referred to as "magnetic flux density". The peak portion of the magnetic flux density in the magnetic flux density distribution will be designated as a magnetic pole. This embodiment has five magnetic poles as shown in FIG. 19. The magnetic pole A confronts the photosensitive drum 3, the magnetic poles B, C, and E are mainly used for conveying the developer 4b, and a magnetic pole D is mainly used for applying the developer 4 to the developing roller 1. Also, the developer restriction member facing section 6 is positioned between the magnetic poles B and C.
FIG. 20 shows, on an enlarged scale, the distribution of the magnetic flux density of the conventional developer restriction member facing section 6. As clearly shown by the drawing, the conventional developing apparatus suffered from a large change in the vertical component of the magnetic flux density on the developing roller 1 in the developer restriction member facing section 6.
Note, the parts to be mounted in the developing apparatus have respective mounting precision errors which must be allowed for in the fabrication. Namely, if the mounting precision for the developer restriction member 2 is .+-.0.5 degrees, if the precision in the magnetized position for the magnet 1b is .+-.3 degrees, if the fixing precision for the magnet 1b .+-.0.5 degrees, and if the processing precision of the other parts is .+-.1 degree, the position of the developer restriction member 2 relative to the magnetic flux density distribution has a precision as shown by the positional precision range Q. The positional precision range Q is generally within the range of .+-.5 degrees of the above-mentioned precision, and becomes 10 degrees in total.
In the conventional developing roller 1, the magnetic flux density in the developer restriction member facing section 6 differs depending on the developing apparatus used, due to the large variation in the magnetic flux density within the positional precision range Q.
As described hereinbefore, the difference in the magnetic flux density causes a variation in the amount of the developer 4 to be carried to the developing section, which has a large affect on the image quality. Accordingly, many different problems arise such as a remarkable difference in the image density depending on the developing apparatus used, a solid, a thin line, a skip in a dots for half torning, and the adherence of the carrier to the photosensitive body.